This invention relates to high intensity discharge lamps and, more particularly, to a DC metal halide lamp comprising a ceramic discharge vessel. In the ceramic discharge vessel, a cathode and an anode are arranged, a discharge space is provided, metal halide and mercury fillings are contained, and a metal heat shield is especially added on the cathode side for maintaining equivalent cold spot temperature in different lamp operating positions.
Most DC metal halide lamps on the market are short arc types such as disclosed in U.S. Pat. Nos. 5,291,100 and 5,144,201 for applications such as projection, vehicle headlamp, fiber optics illuminations, etc. Until now, the majority of metal halide lamps for general lighting have been operated on AC power sources. Due to the constant demand for cost cutting of electronic ballast and the continuous effort on improving efficacy of lighting systems, low wattage DC metal halide lamps fabricated with quartz envelopes for applications in general lighting have been on the market for some time as disclosed in U.S. Pat. No. 4,281,274.
However, the existing DC quartz metal halide lamps on the market are restricted to vertical operation (xc2x145xc2x0). When a DC quartz metal halide lamp, particularly with a formed arc tube, is operated in the horizontal position, the degree of upwards arc bulge increases significantly and creates an unfavorable local hot spot resulting in a significant CCT shift and ultimately shorter lamp life.
It is an objective of the present invention to provide a universal operating DC ceramic metal halide lamp for general lighting applications. With this invention, DC ceramic metal halide lamps utilizing a metal heat shield on cathode side are suitable for universal operation. The DC ceramic metal halide lamp also exhibits relatively better performance over its AC counterpart. Moreover, a full bridge inverter is eliminated in DC electronic ballast so that about 25% component cost reduction for electronic ballast is readily achieved.
There are disadvantages included in the existing AC metal halide discharge lamps and ballast systems:
(a) AC metal halide lamps, such as disclosed in U.S. Pat. Nos. 4,910,432 and 4,935,668 and European patent 0,587,238A1 including quartz and ceramic discharge vessels have identical electrodes that are compromised during both positive and negative cycles. DC metal halide lamps have a unique anode and cathode design that is optimized for operation. This leads to a higher initial efficacy for DC Lamps when compared to AC Lamps with similar color rendering index (CRI) and comparable correlated color temperature (CCT). (see FIG. 4).
(b) There is high re-ignition voltage during initial warm-up and steady state operation if the lamp operates on the traditional magnetic ballast. A ratio of re-ignition voltage to lamp voltage greater than two leads to lamp extinguishing during operation on a conventional lamp supply.
(c) AC electronic ballasts require a full bridge inverter. This not only adds about 25% component cost, but also makes the circuit more complicated compared to the DC electronic ballast.
There are drawbacks in existing DC metal halide discharge lamps with quartz discharge vessel:
(a) The DC quartz lamp for general lighting applications is restricted to vertical operation only. Moreover, the anode has to be located at bottom during operation for performance optimization. If the cathode is located at bottom, a color separation in the arc stream can be visually observed which results from separation of different emitters in the plasma. From a practical point of view, an arc tube has to be mounted differently inside the outer jacket for base up and base down applications. Although the DC quartz lamp exhibits a higher efficacy and a comparable lumen maintenance over AC quartz lamp in vertical operating position, the DC quartz lamp loses the advantage of universal operating which is common for AC lamp.
(b) It is well known by those familiar with the art that IR reflective coating is required for AC quartz metal halide lamps. In DC quartz metal halide lamps, the anode is placed at the bottom for vertical operation. Because the tip temperature of the anode is higher than that of the cathode during operation, the cold spot temperature of the DC quartz lamp is high enough for desirable performance without IR reflective coating on the end bell. The light emitted from the DC arc tube is no longer partially blocked by the IR coating. This also increases the throughput of the DC quartz lamps.
(c) Most commercially available quartz metal halide arc tubes contain sodium iodide as one of their fill ingredients. Sodium can migrate through the quartz wall. The loss of sodium atoms from NaI frees iodine that can then combine with the mercury in the arc tube to form HgI2 which leads to hard starting and a change in color of the lamp. One way to prevent Na loss while the arc is operated on a d.c. power source is to connect a glass sleeve to a point of potential which is positive with respect to the arc tube. In this way, the glass sleeve prevents sodium loss from the arc by trapping ultraviolet light and by shielding the arc from photoelectrons as disclosed in U.S. Pat. No. 4,281,274. However, electrical bias on the glass sleeve brings complexity to the mount structure and ballast circuit design. In contrast to the situation with quartz, migration of sodium through an alumina arc tube wall is negligible. Elimination of Na depletion mechanism in the ceramic,arc tube leads to a smaller initial color spread and a better color stability over lifetime for the DC ceramic lamp.
(d) The anode seal area may exceed the temperature limitations of the molybdenum foil/quartz seal for an equivalent AC electrode length. The temperature of the molybdenum foil to quartz seal of the anode press seal area is expected to be much hotter than the temperature of press seal area with AC electrode which has equivalent length as the anode. With appropriate anode design, the anode seal temperature can be reduced to be suitable for quartz-molybdenum seal. However, it is limited to relatively low current operation so that DC quartz lamp is only suitable for low wattage in general lighting applications.
(e) Due to dimensional non-symmetry of anode and cathode, for lamp wattage of 100 W or higher, different pinch seal setup and process for anode and cathode seal is necessary to ensure the quality of the seal. This adds cost to lamp manufacturing.
A primary objective of the present invention is to provide a ceramic DC lamp which overcomes the disadvantages of a quartz DC lamp.
Another objective of the present invention is to provide a DC lamp which maintains the attributes of low wattage AC lamps (such as universal operation) and yet overcomes the disadvantages of AC operation.
Yet another objective of the present invention is to provide a low wattage metal halide lamp that can be operated with an DC electronic ballast, resulting in substantial system cost reduction.
Still another objective of the present invention is to provide the design features of DC ceramic metal halide lamp for universal burning operation and different correlated color temperature as well as different chemistries.
The objectives are achieved according to the invention by providing a discharge lamp with a ceramic arc tube, preferably with a bulge shape but not limited to one particular shape. There are two electrodes sealed into the arc tube, one tungsten anode with a ball shaped tip and one thoriated tungsten cathode with rod and wound coil. The arc tube is filled with a certain amount of rare gas, mercury, and rare earth chemistry.
Due to the cooling effect of the emitted electrons, the cold spot temperature while the lamp operated with the cathode at the bottom is lower than that with the anode at the bottom. In order to compensate for the reduction of cold spot temperature, a molybdenum or niobium foil heat shield is secured to the cathode side of the arc tube. In this way, the infrared radiation is reflected back to the arc tube and the heat loss at the end of the main arc tube body on the cathode side is reduced significantly. The arc tube maintains equivalent cold spot temperatures in different lamp operating positions.
This invention offers many advantages over all AC metal halide lamp/ballast systems and DC quartz metal halide lamp/ballast systems.
(a) The DC ceramic metal halide lamp has a unique ball shaped anode and coil wound rod cathode. Both electrodes are optimized for direct current operation. The overall lamp performance is demonstrated to be relatively better than AC ceramic lamps.
(b) A metal shield is specifically added on the cathode side to preserve the heat. The metal heat shield minimizes the cold spot temperature variations due to operational position changes. Without the metal shield, the DC ceramic lamp exhibits significant color temperature variation for different operating positions.
(c) DC ceramic lamps with a metal heat shield on the cathode side meet all the universal operating criteria. However, the metal heat shield is not limited to the cathode end only. For various anode design and seal arrangements, a metal tube can be added on the capillary of the anode side for universal burning applications.
(d) Migration of sodium through the alumina arc tube wall is negligible. Alumina hardly dissolves in tungsten so that the reactions of the rare-earth elements with alumina are self-limiting. Therefore, there is minimal depletion of the metallic part of the fill. Reduction of the depletion mechanisms for both Na and other metallic parts guarantees a stable halide composition and thus a small initial color spread and very good color stability over lifetime in the ceramic arc tube.
(e) The ceramic metal halide lamp is much more compact than the quartz lamp. The arc length of DC ceramic lamp is shorter than that of the DC quartz lamp. There is no visual color separation observed when the lamp is operated with cathode at the bottom. The visual color separation in the arc stream can be easily observed for the quartz DC lamp when it operates with the cathode at the bottom.
(f) The metal heat shield covering the cold-spot area of a discharge vessel will block only a small amount of emitted visible light since the area is behind the electrode. However, the overall efficacy of the DC ceramic lamp with heat shield is higher than the equivalent AC ceramic lamp.